The neuropeptide, Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH, (F-8-F-NH 2) was isolated from bovine brain and found to have morphine modulating activity when administered centrally to the rat. In the spinal cord, this peptide is highly concentrated in the dorsal horn where it is stored in nerve terminals in the substantia gelatinosa. In this study, the secretion of F-8-F-NH 2 was investigated.By an in vitro superfusion technique using isolated intact spinal cords,it was found that F-8-F-NH 2 could be released by depolarizing concentrations of KC1 in a calcium dependent manner. This induced release of F-8-F-NH 2 was attenuated by addition of morphine to the superfusion medium. The effect of morphine was found to be dose dependent. The result of this study is in good agreement with the hypothesis that F-8-F-NH 2 may have a modulatory role in opioid mediated antinociception. The proposed course of this study is to investigate the physiological release of F-8-F-NH 2 in normal and morphine tolerant or dependent rats. The physiological secretion of F-8-F-NH 2 by transmitters will also be investigated. The aim of this study is to explore the possible role of F-8-F-NH 2 in the development of opiate dependence or tolerance.